Debian

Debian is a Linux release, which is widespread for desktop machines, however it's been gaining in popularity as a server OS over the last couple of years too. It is very stable and secure, since fixes are released in just a couple of days after a problem is discovered. Debian has a vast support community, so when you have any kind of questions or issues with the OS or with any of the 30,000+ packages it is equipped with, you will be able to contact the developers directly through email using special e-mail lists and receive a reply within 15-20 min. The Debian community supplies the top level of support possible, since all your questions will be handled by the very same people who made Debian in the first place, so they have an exhaustive knowledge of how their own system operates. Because the Operating System is absolutely free to use and distribute, no license costs will be added to your web hosting fees and you can change the code files and any other software which you install without restrictions, in order to optimize the software environment for your web sites. What's more, Debian performs very well on any hardware, as it is light and fast and doesn't need much memory.